Investigating tip-speed ratio balancing for enhanced hydrodynamic performance of counter-rotating marine current turbine

Counter-rotating marine current turbines demonstrate significant potential for tidal power extraction. However, existing studies neglect the critical interaction between upstream (λ1) and downstream (λ2) tip-speed ratios (TSRs) within rotating wake environments. This work systematically investigates TSR balancing strategy via coupled analytical (momentum theory with wake rotation) and numerical methods (ANSYS 2022 R1, SST k-ω turbulence model) across a broad TSR design range (λ1, λ2: 0.5–9.7). Results reveal that the rear rotor exhibits greater sensitivity to TSR changes. Contrary to conventional design paradigms, novel geometries with λ2/λ1 ≤ 1.3 can still achieve a competitive power coefficient, indicating the potential for efficient operation under such conditions and exceeding the performance of a single-rotor configuration. Analytical solutions show that momentum theory underpredicts power performance at high TSRs due to tip losses, while numerical simulations identify optimal TSR intervals, yielding a 9.3% efficiency gain over a single-rotor system. Furthermore, increasing λ1 accelerates the onset of negative torque in the rear rotor, as λ2 approaches its operational limit. This study challenges prevailing assumptions by demonstrating that rear rotor can efficiently operate at elevated TSRs (λ2/λ1) under stabilized axial induction factors, offering actionable insights for optimizing high-output marine energy systems.